1. Field of the Invention
The present invention pertains to handling magnetic tape with a tape drive, and particularly tracking or servo control to ensure that heads of the tape drive correctly follow tracks of information recorded on the magnetic tape.
2. Related Art and Other Considerations
In magnetic recording on tape using a magnetic tape drive, relative motion between a head unit (typically with both a write element and a read element) and the tape causes a plurality of tracks of information to be transduced with respect to the tape. The magnetic tape is typically housed in a cartridge which is loaded into the tape drive. The tape extends between a cartridge supply reel and a cartridge take-up reel. The tape drive typically has a supply reel motor for rotating the cartridge supply reel and a take-up reel motor for rotating the cartridge take-up reel.
After the cartridge is loaded into the tape drive, the tape is extracted by mechanisms in the drive so that a segment of the tape is pulled from the cartridge and into a tape path that travels proximate the head unit. The extraction mechanisms take the form of tape guides which are mounted on trolleys. During the extraction operation, trolley motors move the trolleys along a predefined trolley path, so that the tape guides which surmount the trolleys displace the tape into the tape path as the trolleys travel along the trolley path. When the trolleys reach the full extent of travel along the trolley path, the tape is proximate the head unit. Thereafter the tape can be transported past the head unit, e.g., by activation of a capstan and/or the supply reel and take-up reel motors, depending upon the particular type of transport mechanisms employed.
In a helical scan arrangement, as the magnetic tape is transported the magnetic tape is at least partially wrapped around a rotating drum so that heads (both write heads and read heads) positioned on the drum are contiguous to the drum as the drum is rotated. One or more write heads on the drum physically record data on the tape in a series of discrete tracks of stripes oriented at an angle with respect to the direction of tape travel. As the tape is transported past the head unit, information can be transduced to or from the tape by the tape drive in recording and reading operations, respectively. The data is formatted, prior to recording on the tape, to provide sufficient referencing information to enable later recovery during readout by one or more read heads. Examples of helical scan tape drives are shown, inter alia, in the following U.S. patents (all of which are incorporated herein by reference): U.S. Pat. No. 4,835,628 to Hinz et al.; U.S. Pat. No. 4,843,495 to Georgis et al.; U.S. Pat. No. 5,065,261 to Hughes et al.; U.S. Pat. No. 5,068,757 to Hughes et al.; U.S. Pat. No. 5,142,422 to Zook et al.; and U.S. Pat. No. 5,602,694 to Miles et al. (which discloses a capstanless helical scan tape drive).
It is common in helical scan tape drives to provide some sort of tracking or servo system to ensure that the heads correctly follow the tracks or stripes, e.g., preferably over a longitudinal centerline of the tracks. Some helical scan tape drives sample a servo signal comprising an analog amplitude of a low frequency pattern written into the data format.
Two techniques have been employed to read this servo information. The first technique uses a wide on azimuth head that overlaps the track on either side of the track being followed. The information on either side of the track occurs at different times since subsequent tracks are staggered in helical recording. Because this information is staggered in time, it may be measured separately and compared. When the information on both sides of the track is of the same amplitude the head is on track and no correction is required. If the information is of different amplitude the head is off track, and the tape speed is modified to bring the drive back on track. This method requires an additional servo head that is of the opposite azimuth as the track being followed. The disadvantage of this additional head is not only cost; tracking error is also introduced because the servo head is not the head reading the data and additional error exists between these two heads. It also requires an additional read channel to processes this head.
A second technique is employed in a helical scan tape drive marketed by Exabyte Corporation as the Mammoth(trademark) drive. The method used on the Mammoth drive differs in that rather than use an additional servo head, with the addition of a tracking offset, the servo information is read by using the read heads that read user data. The servo data with this type of system is now off azimuth with respect to the head that is reading it. In order to improve the signal to noise ratio of this servo information a low frequency is employed to construct it, as the off azimuth loss is not as great at low frequency. The disadvantage of this servo is therefore the large amount of overhead encountered in constructing the low frequency servo pattern. Additional servo signal processing circuitry is also required as the gain and bandwidth requirements of the servo are different than those of the read channel.
The disadvantages of certain current implementations are the need to provide analog processing of the servo information, overhead in the format for this information, as well as the additional cost of an additional head (if applicable) and processing circuitry. This analog processing circuitry has the additional disadvantage of being difficult to integrate in a cost-effective manner as mixed mode ASICs can be difficult to develop, and the circuitry, though being large, is not very expensive. Another disadvantage of these schemes is a head wider than the track being followed is required to measure the analog information from the adjacent tracks.
The following disclose magnetic tape drive systems wherein servo control involves determining a difference in reproduction time between servo signals recorded on the magnetic tape: U.S. Pat. No. 4,868,692 to Nakase et al.; U.S. Pat. No. 5,313,346 to Shimotashiro et al.; and U.S. Pat. No. 5,325,246 to Guisinger et al. Yet some of these servo schemes are adversely affected by variation in the rotational speed of the drum or scanner. Another deficiency of some of these servo schemes is incompatibility with the trend toward narrowing track width.
What is needed, therefore, and an object of the present invention, is a servo or tracking technique which is essentially impervious to variation in the rotational speed of the drum or scanner and/or compatible with narrow width tracks.
In a helical scan recording system, magnetic tape is transported by a tape transport proximate a rotating scanner. The scanner has a pair of read heads mounted thereon, e.g., a first read head and a second read head, which travel in a helical direction on the magnetic tape in view of the transport of the tape and rotation of the scanner.
A synchronization detection system determines a first synchronization mark detection time at which a first synchronization mark is read by the first read head from the first track, and a second synchronization mark detection time at which a synchronization mark read by the second read head from the second track. A position error signal generator develops a position error signal (PES) based upon a difference between the first synchronization mark detection time and the second synchronization mark detection time. A servo or transport controller uses the position error signal to develop a servo correction signal for adjusting the position of the read heads.
In accordance with one aspect of the present invention, a first distance separating the sync marks along the helical direction of head travel is sufficiently less than a second distance by which the read heads of the pair are circumferentially separated on the scanner to render the position error signal (PES) immune to any variation in the rotational speed of the scanner.
In accordance with another aspect of the invention, the position error signal generator comprises a coarse position error signal generator; a fine adjustment signal generator; and a combination element. The coarse position error signal generator develops a coarse position error signal based upon a difference between first synchronization mark detection time and the second synchronization mark detection time, the first synchronization mark detection time and the second synchronization mark detection time being dependent upon byte clock resolution. The fine adjustment signal generator generates an adjustment signal to compensate for the first synchronization mark detection time and the second synchronization mark detection time being dependent upon byte clock resolution. The combination element combines the course position error signal and the adjustment signal to obtain the position error signal PES which is used by the servo controller to keep the read heads traveling along a longitudinal centerline of their respective tracks.
Various embodiments of fine adjustment signal generators are provided. In a first example embodiment, the synchronization detection system comprises a first deformatter and a second deformatter. The first deformatter outputs a first bit position signal to the fine adjustment signal generator indicative of a bit position in which the first synchronization mark is detectable. The second deformatter outputs a second bit position signal to the fine adjustment signal generator indicative of a bit position in which the second synchronization mark is detectable. The fine adjustment signal generator of this first example embodiment comprises a comparator which compares the first bit position signal and the second bit position signal to generate a bit offset as the adjustment signal.
In a second example embodiment, the fine adjustment signal generator comprises a ramp voltage circuit which ramps through a range of analog voltage values upon enablement by the first byte clock. Also included in the second example embodiment is a circuit which provides a digital value corresponding to an analog voltage reached in the range when a signal is output from the second byte clock. The digital value being indicative of a delay time, is applied as the adjustment signal.
In a third example embodiment, the fine adjustment signal generator comprises a high resolution delay measurement circuit which includes a chain of gate elements. Upon a transition of a signal from the first byte clock, the gate elements are successively switched from an inactive state to an active state. A predetermined time delay exists between successive activations of gate elements. A transition of a signal from the second byte clock enables the gate elements to output their states to a position detection circuit. The number of gate elements having the active state is indicative of a delay time which is used as the adjustment signal.